Spanish Language Stack Exchange is a question and answer site for students, teachers, and linguists wanting to discuss the finer points of the Spanish language. It's 100% free, no registration required.

In this first sentence as discutir is used in a transitive way and it is an example of passive voice (pasiva refleja) and it would be the same as saying this:

En el coloquio un tema interesante fue discutido

so it could be translated as: "In the symposium an interesting subject was discussed". Here "se" marks that this sentence is a passive voice (pasiva refleja).

On the other hand:

7, En el coloquio se discutió de un tema interesante.

This second sentence is an example of intransitive use of that verb because it's followed by the preposition "de". As it is instransitive then it can't be a passive voice.

I'd say that in this case it is an impersonal sentence. and its meaning would be something like: "In the symposium people discussed an interesting subject". Here "se" marks that this sentence is impersonal.